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Wolf-Rayet Stars Approaching Supernova
Wolf-Rayet Stars are the brightest stars known. They are aged and approaching the end of their lives. Before they die in a violent supernova, they become extremely active and dangerous. For additional details regarding related stars, see these pages: *The H-R Diagram *Wolf-Rayet Stars *O-Type Stars *Red Supergiants Approaching Supernova *Supergiant Stars. Lifeline The life of a star can be divided into a main sequence (when the star is fusing hydrogen) and an aged branch (when the star runs out of hydrogen). The most massive O-Type Stars, with a mass of at least 100 Solar masses, behave completely different then smaller O-type or their smaller relatives, B-Type Stars. They are extremely luminous and have convective currents. Thanks to these currents, the core exchanges matter with the outer envelope of the star. Hydrogen fusion occurs fast and is not interrupted until most of the hydrogen, both in the core and in the outer envelope, is exhausted. Unlike other stars, an inert core of helium is not formed gradually, but fast. Hydrogen fusion flame does not slowly proceed outwards from the core, in a surrounding shell, it continues until there is virtually no hydrogen left except for an outer envelope. Wolf-Rayet Phase A Wolf-Rayet star is a star whose photon radiation pressure overcomes gravity. The star loses mass into space in the form of a powerful solar wind. This happens only after hydrogen is exhausted in the core and helium fusion starts. As hydrogen is exhausted, the core contracts and helium fusion begins. However, this process is highly energetic. Unlike their smaller cousins, the Supergiant Stars, the bright Wolf-Rayet Stars produce far more energy. At some point, photon radiation pressure is so high, that the outer envelope cannot be held in place by gravity and is pushed away into space in the form of a powerful and continuous solar wind. In the first phase of life, the hydrogen shell is expelled. Smaller stars will reach their death in this phase. In case of bigger stars, hydrogen is completely expelled and the helium nucleus becomes visible. Helium fusion transforms helium into carbon and oxygen. In case of the most massive Wolf-Rayet stars, the helium shell is expelled, leaving behind a carbon nucleus. Carbon fusion is more energetic then helium fusion, but lasts far less. In case of massive Wolf-Rayet stars, it might last less then 100 years. Before The End The major difference between Red Supergiants Approaching Supernova and Wolf-Rayet stars approaching supernova is the size of the core. The helium core might be weighting over 40 Solar masses before carbon fusion starts. The carbon core might weight over 30 Solar masses when it is exposed. Nuclear reactions occur much faster and the released heat reaches the surface more easily. Pressure bubbles might be formed in the core and can lead to massive explosions on the surface. Pair-instability events can occur more often and can trigger pre-supernova explosions. Solar winds can become extremely powerful. A star weighting 140 Solar masses can expel half of its mass in the last 10 years of life. Expelled matter consists mostly of carbon and oxygen. So, any existing planet is fried by the solar wind before the actual supernova occurs. In case of a star like Betelgeuse, carbon fusion lasts 600 years, followed by 100 years of pause. Then follow neon fusion (1 to 2 years), oxygen fusion (0.5 to 1 years), followed by a pause of roughly 40 days and silicon fusion (1 to 2 days). In case of Wolf-Rayet stars, carbon fusion might last less then 100 years (even less then 50 years for the most massive ones), followed by an extremely fast succession of other stages. Oxygen fusion can be reduced, for example, to only a few days. The End These stars usually explode in massive Ib type supernovas. These explosions are some of the most violent and leave behind a black hole. They contain larger amounts of heavy elements then other types of supernova. The supernova wave will interact with matter already ejected, producing powerful magnetic and radio waves. Possible Planets If these stars had a planet, it would be already scourged of atmosphere and water before supernova. Since Ib and pair-instability are among the most powerful supernovas known to exist, it is questionable if close planets would have any chance of surviving. The Habitable Zone around O-Type Stars is very far away. If we replace the Sun with such a star, habitable zone would be around Sedna's semi major axis. In case of Wolf-Rayet Stars, habitable zone would be in the Oort Cloud. At that distance, a planet could survive a supernova, but will have its surface scourged of volatiles. Terraforming is impossible, because of the short lifetimes of such stars and because of extreme solar winds. However, a planet could shield scientific instruments, which could survive buried a few km underground. After the supernova, the area will be inaccessible for spaceships, as the debris cloud, hot and moving with high velocities, will behave like a powerful radiation belt. However, after a century, surviving planets could be a good destination for mining companies. Powerful supernovas can produce a lot of heavy elements.